(1) Field of the Invention
This invention relates to the casting of metals, particularly aluminum and aluminum alloys. More particularly, the invention relates to the casting of such metals by direct chill casting techniques.
(2) Description of the Related Art
Metal ingots are commonly produced by direct chill (DC) casting of molten metals by means of which a molten metal is poured into a mold having an open upper end and (after start-up) an open lower end. The metal emerges from the lower end of the mold as a metal ingot that descends as the casting operation proceeds. In other cases, the casting takes place horizontally, but the procedure is essentially the same. Such casting techniques are particularly suited for the casting of aluminum and aluminum alloys. Unfortunately, ingots of certain metals cast in this way may be susceptible to so-called “hot-tearing” (also known as “hot-cracking”) as the ingots emerge from the mold and before they have fully solidified. Hot-tearing means the formation of a crack of critical size at the surface of the ingot following chilling but before full metal solidification. This may be caused by the shrinkage of the metal as the cooling and solidification proceeds and also by the mechanical contribution of thermal stresses. Some alloys are more susceptible to hot-tearing than others, and hot-tears are most prevalent in AlCu alloys (e.g. AA2xxx series aluminum alloys), with the effect being most pronounced at a Cu-content of about 1.4% by weight. Some aluminum magnesium alloys particularly (Al-2.5 wt. % Mg) are also susceptible to hot-tearing.
To minimize hot tearing in such alloys, it is known to add so-called “grain refiners” to the molten metal. Grain refiners decrease the hot-tear sensitivity of the metal by promoting a fine grain structure in the metal as it solidifies. Fine grains dissipate the accumulated stresses during solidification due to their increased number and density. In particular, grain refiners act to increase the number of solidification sites and thus average-out and redistribute the stresses (associated with the shrinkage that takes place with the generation of solid) that accumulate during solidification and that lead to hot-tears. Materials used in this way as grain refiners include AlTi, TiB2, AlBTi, TiCAl and TiC. Such grain refiners may be produced by co-melting metals to produce a master alloy, adding further ingredients if desired, and adding the master alloy to the metal alloy intended for casting. Ti and TiB2 are the most commonly used grain refiners for aluminum alloys. They are usually added to the main alloys in amounts of 0.01 wt. % or more, and the added amounts tend to be at the higher end when casting metals subject to hot-tearing (in contrast to other metals where the grain refiners may be added to produced desired physical properties of the cast alloy). Unfortunately, these materials tend to be relatively expensive and have to be distributed thoroughly throughout the molten metal and are not always as effective as would be desired. Moreover, in some cases, the metallurgy desired for a particular application may not be that produced by the use of grain refiners added to control hot-tearing.
There is therefore a need for an improved way of controlling hot tearing during the DC casting of such metals.